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Architectural 
Acoustics 


Johns-Man  ville 


Incorporate 

:d 

Execu 

tive  Offices, 

New  York 

Akron 

Columbus 

Houston 

Omaha 

St.  Louis 

Albany 

Dallas 

Indianapolis 

Philadelphia 

St.  Paul 

Atlanta 

Dayton 

Kansas  City 

Pittsburgh 

Stockton 

Baltimore 

Denver 

Los  Angeles 

Portland 

Syracuse 

Birmingham 

Des  Moines 

Louisville 

Rochester 

Tacoma 

Boston 

Detroit 

Memphis 

Sacramento 

Toledo 

Buffalo 

Duluth 

Milwaukee 

Saginaw 

Tulsa 

Canton 

El  Paso 

Minneapolis 

Salt  Lake  City 

Washington 

Chicago 

Erie 

Xashville 

San  Diego 

Wilkes- Barre 

Cincinnati 

Great  Falls 

Newark 

San  Francisco 

Youngstown 

Cleveland 

Houghton 

Xew  Orleans 

Seattle 

Havana,  Cuba 

Canadian  Johns-Manville  Co.,  Limited 

Toronto 

Montreal 

Vancouver 

Winnipeg 

Hamilton 

London 

Ottawa 

Windsor 

Copyright  1921,  by  Johns-Manville  Incorporated 


•  .  ••  •    • 


•  •     •  • 

•    "     •    »      • 


IV/1  <.  VLTO 
5? 


Dining  Room,  Harvard  Cluh,  ii<>sti>ri 

Parker,  Thomas  &  Rice,  Boston,  Architects 

Johns-Mnnrille  Acoustical  Correction  installed  behind  tapestries 


Page  Two 


FOREWORD 

1! 


WITH  the  growing  appreciation  of  both  the  possibiHty 
and  the  necessity  of  producing  comfortable  conditions 
of  hearing  in  any  auditorium,  more  and  more  atten- 
tion has  been  given  by  designers,  builders  and  owners  to  the 
problem  of  architectural  acoustics. 

Realizing  this  fact  and  the  importance  of  the  researches  of 
Prof.  Sabine  of  Harvard  University  in  establishing  the  subject 
on  a  true  scientific  basis,  we  undertook  as  long  ago  as  1911, 
to  enlarge  our  service  to  the  architectural  profession  by  making 
the  results  of  these  researches  available  in  a  practical  and  com- 
mercial form.  Until  his  recent  death.  Prof.  Sabine  was  retained 
by  us  in  a  consulting  capacity,  with  active  assistance  from  Mr. 
Clifford  M.  Swan,  formerly  on  the  staff  of  the  Physics  Depart- 
ment of  the  Massachusetts  Institute  of  Technology,  who  was  for 
many  years  closely  associated  with  Prof.  Sabine  and  who  was, 
in  fact,  his  only  student  in  the  subject.  Mr.  Swan  is  now  con- 
tinuing the  work  of  Prof.  Sabine  and  we  have  arranged  to  have 
Mr.  Swan's  counsel  and  advice,  as  formerly. 

Thus  with  the  best  expert  assistance,  and  with  the  new 
materials  and  the  perfected  methods  that  we  have  developed 
during  the  past  ten  years,  we  are  able  to  offer  our  clients  the 
benefits  of  the  latest  scientific  acoustical  discoveries,  together 
with  a  technique  of  construction  acquired  through  wide  and 
varied  experience  in  this  type  of  work. 


Page  Three 


46^786 


Federal  Court  Room,  Indianapolis 

Rankin,  Kellogg  &  Crane,  Philadelphia,  Architects 

Johns-Manville  Acoustical  Correction  installed  in  wall  and  ceiling  panels 


In  the  following  article  Mr.  Swan  presents  a  brief  discussion 
of  the  subject  of  architectural  acoustics,  outlining  the  principal 
problems  which  are  encountered  and  the  lines  along  which  they 
are  properly  solved.  We  have  added  a  number  of  illustrations 
showing  some  of  the  buildings  in  which  we  have  installed  acous- 
tical treatment. 

This  book  is  presented  to  architects  with  our  compliments, 
in  the  hope  that  in  it  they  may  find  the  possibility  of  solu- 
tion of  many  acoustical  problems.  Our  services  are  always  at 
their  disposal  without  fee  for  examinations  and  reports,  and 
we  are  ready  to  furnish  estimates  and  to  execute  corrective 
work  promptly  and  efficiently  in  any  section  of  the  country. 

Johns-Manville 

Incorporated 


Page  Four 


«A 


■...•: 


Architectural 
Acoustics 


Reprinted  by  permission  from  the  Journal  of  the  American  Institute 
of  Architects,  Issue  of  December,  1919 

CLIFFORD  M.  SWAN,  S.B.,  A.M. 


OF  THE  many  technical  problems  with  which  an  archi- 
tect is  from  time  to  time  confronted,  not  the  least  im- 
portant is  that  of  architectural  acoustics.  In  churches, 
courtrooms,  lecture  halls,  and  the  great  auditoriums  of  monu- 
mental buildings,  it  ranks  equal  in  importance  with  lighting, 
heating  and  ventilation.  Strange  to  say,  only  within  the  past 
twenty  years  has  there  been  developed  a  definite  science  ap- 
plicable to  the  design  and  construction  of  buildings,  as  well  as 
effective  materials  for  correction  and  a  technique  of  their  in- 
stallation. 

It  is  remarkable  that  in  no  other  field  of  technical  knowledge 
have  antiquated  ideas  yielded  so  slowly  to  oft-repeated  experi- 
ence. At  last,  however,  through  the  results  of  scientific  inves- 
tigation, the  useless  stringing  of  wires  is  being  discontinued, 
while  the  sounding-board,  moderately  useful  at  times,  is  not 
now  considered  an  universal  panacea,  and  simple  ratio  between 
length,  breadth  and  height  is  no  longer  looked  upon  as  an 
adequate  basis  for  acoustical  design. 

Broadly,  the  subject  of  architectural  acoustics  may  be  divided 
into  two  parts:  The  transmission  of  sound  through  floors,  walls, 
or  other  partitions  into  rooms  situated  apart  from  the  source; 
and  the  phenomena  arising  within  a  given  room  or  auditorium 
due  entirely  to  factors  of  interior  design  and  finish  as  affecting 


Page  Five 


^^ 


.ft^m^ 


House  of  Representatives,  Missouri  State  Capitol,  Jefferson  City 

Tracy  &  Swartivout,  iSew  York  City,  Architects 

JohnS'Manville  Acoustical  Correction  installed  behind  draperies,  and  on  ceiling 


sound,  either  generated  within  the  room  or  entering  from 
without. 

SOUND  TRANSMISSION 

The  first  of  these  divisions,  that  of  sound  transmission,  is 
but  just  beginning  to  receive  scientific  attention,  and  the 
amount  of  reliable  data  at  hand  is  small.  The  problem  of  dead- 
ening walls  or  floors  is  quite  as  much  a  question  of  construction 
as  of  deafening  materials.  The  importance  of  the  former  ele- 
ment has  not  been  appreciated,  while  even  in  the  case  of  mate- 
rials themselves,  trustworthy  figures  are  lacking  as  to  their 
efficiency. 

The  common  practice  in  the  past  seems  to  have  been  to  de- 
sign a  material  with  especial  reference  to  its  heat  insulating 
qualities  and  then  arbitrarily  to  assume  corresponding  proper- 


United  States  Court  of  Appeals,  Denver 

Tracy  &  Swartwout,  New  York  City,  Architects 

Johns-Manville  Acoustical  Correction  installed  behind  wall  draperies 


ties  for  sound  insulation,  an  assumption  wholly  unjustifiable 
both  in  theory  and  practice.  Such  success  as  may  have  been 
attained  in  actual  installations  is  to  be  attributed  to  practical 
experience  in  work  of  this  character  which  has  shown  some 
of  the  pitfalls  to  be  avoided,  rather  than  to  definite  and  exact 
information.  In  the  present  state  of  the  subject,  no  one  can 
predict  with  assurance  the  degree  of  efficiency  to  be  obtained 
from  any  given  form  of  construction  or  material.  Research  is 
however,  progressing  along  these  lines,  and  the  time  is  doubtless 
not  far  distant  when  at  least  the  principal  factors  in  the  prob- 
lem will  be  capable  of  analysis. 

AUDITORIUM  ACOUSTICS 
In  marked  contrast  to  the  uncertainty  surrounding  the  trans- 
mission problem,  is  the  definiteness  attaching  to  the  other  great 


Page  Seven 


Waiting  Room,  Southern  Pacific  Railroad,  Oakland,  California 

Jarvis  Hunt,  Chicago,  Architect 

Johns-Manville  Acoustical  Correction  installed  in  wall  and  ceiling  panels 


division  of  architectural  acoustics,  that  of  the  so-called  in- 
ternal acoustics  of  an  auditorium.  In  this  field,  much  careful 
scientific  investigation  has  been  done,  notably  by  the  late  Prof. 
Wallace  C.  Sabine,  of  Harvard  University,  which  merits 
thoughtful  consideration  by  all  designers  or  owners  of  large 
auditoriums.  Sabine's  researches  have  been  published  from 
time  to  time  in  the  architectural  and  engineering  press  and 
have  been  supplemented  by  the  work  of  Jaeger  and  others. 
They  form  the  basis  of  all  modern  practice.  The  result  has 
been  to  make  possible  not  only  the  correction  of  existent  audi- 
toriums but  also  the  design  and  construction  of  new  ones  in  a 
manner  such  as  to  obviate  all  difficulties  from  the  outset. 

While  much  of  the  success  to  be  attained  in  this  direction 
must  of  necessity  be  a  matter  of  expert  consideration  and  advice, 


Page  Eight 


Music  Building,  Harvard  University,  Cambridge 

Howells  &  Stokes,  New  York  City,  Architects 

Johns-Manville  Acoustical  Correction  installed  in  wall  panels 


on  account  of  the  complexity  and  inter-relation  of  the  various 
phenomena  involved,  yet  the  various  factors  entering  into  the 
problem  should  be  a  matter  of  common  knowledge  in  order 
that  there  may  be  a  due  realization  of  their  importance  and  pos- 
sibilities of  control. 

The  phenomena  influencing  the  acoustics  of  an  auditorium 
may  be  classified  under  the  heads  of  reverberation,  echo,  reso- 
nance, and  interference,  and  these  must  be  studied  with  regard 
to  their  effects  upon  the  distinctness  and  loudness  of  speech, 
and  the  tonal  quality  of  music.  As  is  to  be  expected,  they  are 
not  entirely  independent  one  of  another;  nor  are  they  wholly 
unmitigated  evils  in  themselves,  their  presence  to  a  limited 
extent  often  being  necessary  and  desirable  if  not  carried  to  an 
extreme.     The  essential  factors  which  influence  and  control 


Pcge  Nine 


Pool,  Ida  Noyes  Gymnasium,  Vniversity  of  Chicago,  Chicago 

Coolidge  &  Hodgdon,  Chicago,  Architects 

Johns -Manville  Acoustical  Correction  installed  in  ceiling  panels 


them  are  the  size  and  shape  of  the  chamber,  the  contour  of  the 
interior  surfaces,  the  nature  of  the  construction  and  finish,  the 
amount  and  kind  of  furnishing  and  the  number  of  persons  pres- 
ent, in  so  far  as  these  factors  affect  the  reflection,  diffraction, 
and  absorption  of  sound.  Refraction,  caused  by  currents  of 
air  of  different  densities  arising  from  the  heating  and  ventilat- 
ing, may  conceivably  become  a  factor,  but  has  been  shown  to  be 
negligible  under  almost  all  circumstances. 

REVERBERATION 

The  most  common  source  of  trouble  is  reverberation,  more 
so  today  than  formerly,  owing  to  modern  fireproof  construc- 
tion. In  the  technical  sense,  reverberation  signifies  the  pro- 
longation of  a  sound  by  its  multiple  reflection  from  surface  to  sur- 


pass Ten 


Fourth  Church  of  Christ,  Scientist,  Cleveland 

Briggs  &  Nelson,  Cleveland,  Architects 

Johns-Manville  Acoustical  Correction  installed  in  wall,  cove  and  ceiling  panels 


face  before  its  energy  is  sufficiently  absorbed  to  become  inaud- 
ible. Since  the  average  sound  must  be  reduced  approximately 
to  one-millionth  of  its  original  intensity  before  it  reaches  the 
limit  of  audibility,  and  since  such  a  sound  once  produced  in  a 
bare  room  loses  but  from  2  to  4  per  cent,  of  its  energy  at  each 
reflection,  it  is  evident  that  such  a  sound  must  be  reflected 
several  hundred  times  before  it  becomes  inaudible.  Since  this 
projcess  consumes  time,  owing  to  the  finite  velocity  of  propaga- 
tion, the  sound  is  prolonged  for  a  period  of  several  seconds  after 
the  original  source  has  ceased  to  emit  energy. 

The  period  of  reverberation  is  evidently  inversely  propor- 
tional to  the  absorbing  power  of  the  room,  and  directly  propor- 
tional to  the  size,  since  the  distance  traveled  by  the  sound  wave 


Page  Eleven 


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^^^^^^^^B^^^^ 

Chapel  of  the  Intercession,  Setv  York  City 

Cram,  Goodhue  &  Ferguson,  Sew  York  City,  Architects 

Johtu-Manville  Acoustical  Correction  installed  in  ceiling  panels 


Pagi 


Fourth  Presbyterian  Church,  Chicago 

Cram,  Goodhue  &  Ferguson,  Boston,  Architects,  Howard  Shaw,  Chicago,  Assoc.  Architect 

Johns-Manville  Acoustical  Correction  installed  in  ceiling  panels 


Page  Thirteen 


I 


between  reflections  is  greater  the  larger  the  room.  Its  effect, 
if  present  in  excessive  amount,  is,  as  noted,  to  prolong  every 
sound  to  such  an  extent  as  to  cause  an  overlapping  and  blurring 
which  is  especially  distressing  in  the  case  of  speech.  For  this 
reason,  in  an  auditorium  intended  for  speaking  alone,  it  should 
be  reduced  as  far  as  is  consistent  with  the  carrying  power  of  the 
sound.  It  is  important  to  bear  in  mind,  however,  that  as  the 
reverberation  is  reduced,  the  loudness  is  diminished  in  very 
nearly  the  same  ratio,  so  that  there  is  necessarily  a  limit  to  the 
amount  of  permissible  absorption.  In  the  case  of  an  audito- 
rium used  for  music,  a  greater  amount  of  reverberation  is  desir- 
able than  in  one  used  for  speaking  only,  in  order  that  the  over- 
tones may  not  be  obliterated  (high  pitches  being  more  readily 
absorbed  than  low)  and  the  quality  of  tone  destroyed.  The 
time  of  decay  of  a  sound  can  be  calculated  with  a  considerable 
degree  of  precision,  and  the  proper  design  or  treatment  laid 
out  which  will  reduce  the  reverberation  to  the  amount  best 
suited  for  the  room  under  consideration. 

The  amount  of  reverberation  can  be  controlled  in  various 
ways.  As  already  shown,  it  decreases  with  the  size  and  height 
of  the  room,  other  factors  remaining  constant.  It  is  also  dimin- 
ished by  the  presence  of  recesses  or  balconies  designed  to  hold  a 
number  of  people  or  heavily  upholstered  seats.  Draperies, 
carpets,  furniture,  and  upholstery  are  all  effective  in  proportion 
to  their  absorbing  power,  lined  and  heavy  materials  being  nat- 
urally much  more  efficient  than  light  and  thin  ones.  The  cloth- 
ing of  an  audience  is  an  important  factor,  being  nearly  totally 
absorptive  of  sound,  and  many  an  auditorium  which  is  exceed- 
ingly bad  when  empty  is  entirely  satisfactory  when  filled  with 
people.  Oftentimes,  architectural  or  other  considerations  limit 
the  design  or  furnishings  so  that  a  correct  condition  is  not  at- 
tained, no  matter  what  the  number  in  attendance.  Under 
such  circumstances,  recourse  must  be  had  to  a  modification  of 
the  materials  used  as  an  interior  finish. 


Page  Fourti 


3         »>,»»»  J 

J  J      >'      >      >        3       J 


J  J         >         J      »  >  > 


Amasa  Stone  Chapel,  Western  Reserve  Lniversily,  Cleveland 

Henry  Vaughan,  Boston,  Architect 

Johns-Manville  Acoustical  Correction  installed  in  ceiling  panels 


Page  Fifteen 


I 


Great  Hall.  College  of  the  City  of  New  York,  New  York  City 

Ceorpe  B.  Post  &  Sons,  New  York  City,  Architects 

Johns-Manville  Acoustical  Correction  installed  in  ceiling  and  rear  wall  panels 


Page  SueLxi 

I 


CORRECTIVE  MATERIALS 

The  most  common  method  of  producing  a  sufficient  degree  of 
absorption  is  to  place  upon  portions  of  the  walls  or  ceiling  a  cer- 
tain amount  of  felt  of  an  extent  so  chosen  as  to  provide  the 
requisite  absorbing  power  for  sound.  There  are  various  kinds 
of  felt  on  the  market,  but  of  these,  two  have  been  found  to  be 
especially  suited  to  this  class  of  work.  One  is  a  yellow  jute 
felt  such  as  is  used  for  saddle  lining,  and  the  other  is  a  special 
type  of  hair  felt  with  gauze  reenforcement  and  freed  from  the 
usual  impurities  found  in  cattle  hair.*  Both  of  these  materials 
are  efficient  absorbers  of  sound.  As  in  all  substances,  the  degree 
of  absorption  varies  with  the  pitch  of  the  sound.  Both  grades 
carry  a  low  percentage  of  oil,  are  durable,  vermin-proof  and 
non-inflammable,  a  combination  of  qualities  in  conjunction 
with  their  high  absorbing  power  which  is  not  to  be  found  in 
most  other  felt  and  which,  therefore,  renders  them  particularly 
suitable  for  acoustical  requirements. 

Such  surfaces  as  receive  the  felt  treatment  must  afterward 
be  covered  and  protected  with  a  decorative  fabric  or  membrane, 
and  this  fabric  must  be  chosen  with  care  in  order  that  it  may 
not  too  greatly  detract  from  the  absorbing  power  of  the  felt  be- 
neath. The  use  of  a  painted  membrane  or  simply  a  dyed  cloth 
is  an  important  question,  as  the  absorbing  power  of  the 
treatment  depends  on  this  factor  both  in  absolute  amount 
and  variation  with  the  pitch.  Ordinary  paints  diminish  the 
absorption  considerably  but  special  fabric  coatingsf  have  been 
devised  which  affect  it  only  to  a  slight  extent.  In  many  cases, 
beautiful  effects  can  be  obtained  by  covering  the  felt  with  tap- 
estry hangings  or  painted  cloth  panels  of  similar  design,  such 
as  were  used  in  the  Little  Theatre  of  New  York  and  the  great 
hall  of  the  Harvard  Club  of  Boston.     In  all  cases  the  form  and 

*  This  material  is  Johns-Manville  Akoustikos  Felt   used  by  our  company  in  all  acoustical 
correction  work. 

t  Johns-Manville  Akousto-Lite  Fabric  Coating. 


Page  Seventeen 


hirst  i.ungregational  Chunk,  Monlcluir.  A'.  /. 

Bertram   Grosvenor  Goodhue.  .V«?i«'  York  City,  Architect 

Rumford  Tile  used  as  finish  on  walls  and  vaults 

Constructed  by  R.  Guastavino  Co.,  New  York  City 


Page  Eigktee 


manner  of  decoration  must  be  studied  with  relation  to  the 
architectural  and  acoustical  requirements  of  the  individual 
problem. 

The  location  of  treatment  with  such  absorbing  materials  is  of 
prime  importance,  not  only  because  their  efficiency  in  reducing 
reverberation  depends  on  their  receiving  the  major  portion 
of  the  reflected  waves  of  sound,  but  also  on  account  of  certain 
phenomena  of  localized  effect  to  be  discussed  presently.  A  care- 
ful study  is,  therefore,  necessary  in  all  cases  to  determine  the 
extent  and  location  of  those  areas  best  suited  to  an  effective 
treatment,  both  from  an  acoustical  and  an  architectural  stand- 
point. 

An  important  contribution  has  recently  been  made  to  acous- 
tical science  in  the  development  of  two  patented  materials  of 
structural  character  suitable  for  the  interior  finish  of  new  build- 
ings and  having  a  high  degree  of  absorption  comparable  in  effi- 
ciency to  felt  of  equal  thickness.  One  is  a  ceramic  tile*  and  the 
other  is  an  artificial  stone. f  Both  are  manufactured  in  such  a 
way  as  to  produce  intercommunicating  pores  of  uniform  size 
which  give  the  maximum  sound  absorption.  These  materials 
are  not  plastic,  but  must  be  moulded  and  set  in  mortar  as  a 
masonry  finish.  They  are  especially  adapted  to  tile  arch  con- 
struction, and  on  account  of  their  combination  of  acoustical, 
fireproof,  and  structural  qualities  furnish  new  and  useful  possi- 
bilities in  architectural  design  and  construction. 

SOUNDING  BOARDS 

The  question  is  sometimes  asked  as  to  the  value  of  a  sounding- 
board  in  improving  acoustical  conditions.  Such  a  device,  if 
properly  designed  and  placed,  causes  a  slight  reduction  in  the 
amount  of  reverberation  when  a  speaker  stands  beneath  it,  as  it 
casts  a  certain  amount  of  sound  shadow  on  the  ceiling,  thus  con- 

*  "Rumford  Tile"  t  "Akoustolith" 


Page  Nineteen 


Cathedral  of  The  Madeleine,  Salt  Uike  City 
John  T.  Comes,  Pittsburgh,  Architect 
Johfu-Manville  Acoustical  Correction  installed  in  ceiling  panel* 


Page  Twenty 


fining  the  sound  in  some  degree  to  the  lower  portion  of  the  audi- 
toriuip.  In  this  way  by  intensifying  the  sound  reaching  the 
audience,  it  has  its  maximum  effect,  rather  than  in  the  slight 
reduction  in  reverberation.  The  best  form  of  sounding-board 
is  a  plane  surface,  hung  horizontally  near  the  speaker's  head, 
and  of  as  large  extent  as  is  consistent  with  appearance,  the  object 
being  to  subtend  as  large  an  angle  as  possible  at  the  speaker's 
mouth.  The  use  of  parabolic  sounding-boards  is  to  be  de- 
plored, as  they  not  only  produce  unpleasant  focusing  effects  but 
are  also  most  unsightly.  In  any  case,  the  slight  advantage  to 
be  gained  by  the  use  of  a  sounding-board  hardly  compensates 
for  its  expense  and  appearance. 

EFFECTS  OF  REVERBERATION 

The  phenomenon  of  reverberation,  besides  causing  overlap- 
ping of  syllables  with  consequent  blurring,  also  prolongs  and 
intensifies  any  sounds  entering  from  outside  or  caused  by  shuf- 
fling of  feet,  coughing,  or  general  restlessness  on  the  part  of  an 
audience.  The  noise  from  such  sources  may,  in  a  reverberant 
room,  be  entirely  sufficient  to  obliterate  a  speaker's  voice.  This 
is  another  reason  for  reducing  the  reverberation,  even  though 
under  quiet  conditions  the  speaker  can  be  heard  with  a  fair  de- 
gree of  comfort. 

In  the  case  of  offices  and  banking-rooms,  especially  those  of 
modern  construction,  the  accumulation  of  sound  energy,  due  to 
the  reverberation  of  noises  from  typewriters,  adding  machines, 
telephone  bells  and  the  like,  is  so  great  as  not  only  to  be  annoying 
but  actually  responsible  for  nervous  fatigue  and  loss  of  efficiency 
of  the  employees.  The  same  phenomenon  is  also  to  be  observed 
in  restaurants,  corridors  of  public  buildings,  and  similar  places. 
An  astonishing  degree  of  quiet  can  be  produced  in  all  such  cases 
by  suitable  absorptive  treatment.  Not  only  is  the  sound  gen- 
erated in  such  rooms  lessened  in  intensity  to  a  marked  degree, 
but  also  that  coming  through  doors  and  windows,  such  as  the 


Page  Twenty-one 


Chapel,  Leland  Stanford,  Jr.,  Univi-n,it\ .  I'aL)  Alto,  California 

■     Ward  &  Blohme,  San  Francisco,  Architects 
Johns-Manville  Acoustical  Correction  installed  in  ceiling  panels 


Page  Twem 


noise  of  traffic.  A  few  decades  ago  such  problems  commanded 
but  little  attention,  but  modern  office  machinery  and  the  hard 
and  non-absorptive  materials  used  in  fireproof  construction 
have  conspired  to  make  the  ordinary  business  office  almost  as 
noisy  as  the  proverbial  boiler-shop.  The  increase  in  errors 
and  the  loss  of  efficiency  on  the  part  of  the  clerical  force  due  to 
this  nerve-shattering  racket  are  facts  now  being  considered  by 
the  welfare  departments  of  many  of  the  most  progressive  busi- 
ness houses. 

ECHO 

While  reverberation  is  doubtless  the  most  frequent  and  easily 
recognized  acoustical  defect,  yet  there  are  other  phenomena 
which  may  be  quite  as  troublesome,  chief  among  these  being 
echo  and  interference. 

Echo  is  due  to  reflection  from  those  surfaces  whose  contour 
and  arrangement  are  such  as  to  bring  the  sound  waves  to  a 
focus.  If  the  source  of  sound  is  short  and  sharp,  and  if  the  dif- 
ference in  path  between  the  reflected  and  direct  waves  is  suffi- 
ciently great,  the  image  produced  by  the  reflected  waves  ap- 
pears as  a  distinct  repetition  of  the  original  sound.  If,  how- 
ever, the  difference  of  path  is  such  that  the  direct  and  reflected 
waves  are  not  separated  but  overlap  on  reaching  the  ear,  or  if 
the  contour  of  the  reflecting  surfaces  is  such  as  to  produce  a 
blurred  and  not  a  sharp  focus,  confusion  is  created  at  that  point 
and  hearing  made  difficult.  Such  effects  are  purely  local  in 
their  character  and  are  not  general  throughout  the  auditorium 
as  in  the  case  of  reverberation.  They,  in  common  with  true 
interference  phenomena,  are  responsible  for  the  "dead  spots" 
of  which  complaints  are  so  often  heard. 

Echo  of  both  types  is  caused  in  a  large  degree  by  curved 
surfaces  such  as  domes,  vaults,  coves,  pendentives,  and  warped 
areas.  It  can  often  be  eliminated  by  due  care  in  design,  either 
by  proper  choice  of  curvature  or  by  deep  coffering.  Some- 
times a  special  form  of  absorptive  treatment  becomes  necessary, 
the  absorbent  areas  being  so  distributed  as  to  produce  half- 


age  Twenty-three 


wave  phase  differences  between  different  portions  of  the  reflected 
wave,  thus  causing  destructive  interference  at  the  focal  point. 

RESONANCE  AND  INTERFERENCE 

Distinct  from  the  reflection  phenomena  thus  far  considered, 
there  exist  more  complex  sources  of  difl^culty  which  may,  in 
general,  be  classed  under  the  heads  of  resonance  and  interfer- 
ence. By  resonance  we  mean  the  sympathetic  vibration  of  the 
body  of  air  within  a  room  in  response  to  some  definite  pitch. 
Its  effect  is  to  magnify  the  intensity  of  the  given  sound,  so  that 
the  latter  is  thrown  into  relief  against  all  other  tonalities,  thus 
producing  an  unbalanced  effect  on  musical  values.  It  is  fre- 
quently encountered  in  organ  chambers,  and  sometimes  even  in 
the  auditoriums  themselves. 

A  more  frequent  source  of  trouble  is  to  be  found  in  difficulties 
arising  from  interference.  When  a  sustained  tone  of  constant 
pitch  is  produced  in  a  room,  the  waves  reflected  from  various 
surfaces  meet  one  another  and  the  oncoming  wave  in  various 
phase  relations,  producing  points  of  maximum  or  minimum 
loudness,  according  as  the  waves  meet  in  the  same  or  opposite 
phases.  Such  regions  may  be  readily  observed  in  a  church  by 
walking  about  the  auditorium  while  a  single  note  is  sustained 
on  the  organ.  The  points  of  maximum  loudness  will  shift  their 
location  as  the  pitch  of  the  note  is  changed,  and  this  becomes  an 
important  factor  for  consideration  in  the  proper  voicing  of  an 
organ.  Such  effects  produce,  as  a  rule,  but  little  disturbance  for 
speech,  but  may  be  very  disturbing  for  music  whenever  tones  are 
sustained  for  more  than  one-tenth  of  a  second.  The  effects  are 
complex  and  require  individual  study  in  every  case  which  arises. 

In  r^sum^,  the  acoustical  quality  of  an  auditorium  depends 
on  shape,  size,  and  material.  If  two  of  these  three  factors  are 
determined,  the  desired  acoustical  result  can  be  secured  only  by 
correspondingly  adapting  the  third.  In  the  completed  build- 
ing, or  with  plans  in  which  the  form  is  fully  determined  by 
other  considerations,  the  adjustment  must  be  by  the  material, 
its  quality,  and  its  efficient  position. 


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LD  21-50m-l. 


464786 


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